Scientists know masks limit the coronavirus’s spread, but it’s impossible for randomized trials to prove that

From the article:

No matter how much time they had, researchers wouldn’t be able to provide a precise understanding of how masks limit contagion because the primary purpose of masks for the community is to stop the transmission of the disease to others, regardless of how well it may or may not protect the wearer.

[...]

Randomized trials look at the benefits of the intervention only for the person who is enrolled in the trial. It’s not possible for them to conclude how effectively masks prevent community spread to others — people who are not enrolled in the trial.

[...]

Scientists would also have to require cloth masks for one group, while forbidding cloth masks for the other. But when people see others wearing masks distributed by researchers, they will improvise and make some masks for themselves. Researchers would have to require and ensure that people in the mask group wore masks everywhere, while surveilling the other group to ensure no one wore a mask at all. This is not only grossly unethical; it also just isn’t possible in any experimental conditions I can imagine.

[..]

We do, however, have important evidence from lab studies. When we understand the underlying mechanisms — that aerosols and droplets that can carry the virus are emitted when people talk, sing, shout or even breathe — it is less necessary to rely on clinical trials to be confident of a benefit. Do cloth masks lessen the chance of these respiratory particles’ being emitted into the air around us, where they could infect others? The answer from multiple studies is a clear and resounding yes.